1. Field of the Invention
The present invention relates to a shift control system/method for shifting a fully or partially automated mechanical transmission in a vehicle equipped with a driveline retarder located interposed between the clutches of the transmission and the vehicle final drive mechanism. In particular, the present invention relates to a shift control system/method of the type described for minimizing or eliminating the tendency of such vehicles to undesirably accelerate during a shift transient if shifting on a downgrade.
2. Description of the Prior Art
Mechanical change-gear vehicular transmissions which utilize positive jaw clutches, of both synchronized and non-synchronized types, to selectively engage and disengage target gear ratios are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 3,105,395; 3,799,022; 4,754,665; 5,086,897; 5,285,694 and 5,370,013, the disclosures of which are incorporated herein by reference.
Vehicular fully or partially automated mechanical transmission systems also are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,361,060; 4,595,986; 4,648,290; 4,736,109; 5,335,566 and 5,415,604, the disclosures of which are incorporated herein by reference.
While widely used and highly commercially successful, a potential drawback of mechanical transmissions is that at least a momentary torque break will occur during manual and/or automatic shift transients as the jaw clutches are disengaged and then engaged and/or as the master friction clutch and/or a torque converter disconnect clutch is disengaged and then reengaged. If a shift into a target gear ratio is attempted on a downgrade, especially a steep downgrade, and the vehicle gains speed during the torque break portion of the shift transient, this can be somewhat annoying and/or disconcerting to a vehicle operator